The present invention relates generally to improving the operability of energy harvesting devices, and more particularly to low voltage start up circuitry for a boost converter in an inductive energy harvesting system.
Engineers have attempted to design “ultra low” power integrated circuits, for example integrated circuits that require extremely low amounts of operating current and which can be operated without being plugged into conventional AC power systems. Instead, it is desirable that such ultra-low-power integrated circuits be powered by small amounts of power “scavenged” or “harvested” from ambient solar, vibrational, thermal, and/or biological energy sources by means of micro-energy “harvesting devices” and stored in batteries or super-capacitors.
Prior Art FIG. 1 shows a conventional ring oscillator 1-1. Ring oscillator 1-1 has an oscillation frequency determined by the values of resistor R0 and capacitors C0 and C1. For a CMOS implementation, the duty cycle of ring oscillator 1-1 is 0.5, i.e., 50%, if the threshold of inverter I1 is midway between the upper supply voltage (typically VDD) and the lower supply voltage (typically ground).
Prior Art FIG. 2 shows a ring oscillator 1-2 which is a modified version of ring oscillator 1-1 of Prior Art FIG. 1. In oscillator 1-2, the output of inverter I2 is connected by conductor 13 to one plate of capacitor C0, the gate of a P-channel transistor M0, and the gate of a N-channel transistor M1. The other plate of capacitor C0 is connected by conductor 15 to the input of inverter I1 and one plate of capacitor C1, the other plate of which is connected to ground. The output of inverter I1 is connected to the input of inverter I2. The drain of transistor M0 is connected by resistor R1 to conductor 15, and the source of transistor M0 is connected to VDD. The drain of transistor M1 is connected to conductor 15 by resistor R2, and the source of transistor M1 is connected to ground.
Thus, in FIG. 2 the resistance is separated into two separate resistors R1 and R2 connected in series between transistors M0 and M1. The total resistance should be equal to R1+R2=2×R0 for oscillator 1-2 of FIG. 2 to have the same oscillation frequency as oscillator 1-1 of FIG. 1. In this case, the frequency of oscillator 1-2 of FIG. 2 is the same that of oscillator 1-1 1 of FIG. 1 but the duty cycle is determined by the resistor ratio R0/R1), and therefore can be set to any desired value.
During start-up operation in an energy harvester, the supply voltage VDD supplied to the oscillator of a start-up circuit for a DC-DC boost converter (which converts a DC output or a rectified output of the energy harvester to a battery charging voltage) is very low, approximately 0.4 volts. Consequently, none of the circuitry in the boost converter is operable during the start-up operation. Setting the duty cycle of the oscillator in FIG. 2, when it is used in a start-up circuit for the DC-DC boost converter, by setting a ratio of resistors R1 and R2 in FIG. 2 is not adequate if the input voltage has different values and varies over a wide range, e.g. from 0.4 volts V to 2.0 volts. This is because the duty cycle preferably is equal to the ratio of input and output voltages of the boost converter, and therefore the duty cycle should be adjusted as the output voltage of the boost converter rises during charging of the load capacitance. The duty cycle also should be adjusted as the boost converter input voltage varies when its output voltage remains stable.
A DC-DC boost converter should be able to start up in response to an input voltage VIN as low as 0.4 volts in the absence of a charged-up battery or any other energy harvester power source. For example, the minimum workable value of input voltage VIN of a DC-DC boost converter needs to be approximately 0.4 to 0.5 volts in order to boost the output of a single solar cell harvester. However, until the output voltage of an energy harvesting device applied to provide the input voltage of a boost converter reaches a value of approximately 1.3 to 1.5 volts, none of the usual control circuitry inside the boost converter is operable. As a practical matter, meaningful feedback can not be produced by the boost converter to control the duty cycle of its switch transistor until the output voltage of the boost converter is greater than approximately 1.6 to 1.8 volts.
The closest prior art is believed to also include U.S. Pat. No. 7,081,739 entitled “Voltage Converting Circuit Having Parallel-Connected Switching Devices” issued Jul. 25, 2006 to Osinga et al. During start-up the power switch of the disclosed boost converter is toggled on and off without feedback, and is controlled only by a low-voltage start up oscillator. The duty cycle of the oscillator is chosen for the worst-case combination of low input voltage from a solar collector, low inductor value, and load resistance in order to provide sufficient current to cause the output voltage to rise to a value at which normal feedback operation of the converter can start. However, that choice of duty cycle leads to over-designing of the power switch and too much consumption of current through the inductor of the boost converter during start up operation in most modes of operation.
It should be noted that even short-term overloading of the inductor of a boost converter may cause failure of the inductor, especially when the inductor is implemented as a low-cost monolithic inductor.
Thus, there is an unmet need for a low-cost, low complexity, low power start up circuit and method for use in conjunction with a boost converter which has a very low input voltage, especially for use in energy harvesting applications.
There also is an unmet need for a low-cost, low complexity, extremely low power start up circuit and method for use in conjunction with a boost converter having a very low input voltage and which avoids damage caused by excessive current in the inductor and/or power switch transistor of the boost converter.
There also is an unmet need for a way to avoid over-design of the power switch in a DC-DC boost converter, especially in energy harvesting applications.
There also is an unmet need for a start up circuit technique which is capable of starting up a DC-DC boost converter from an input voltage that is substantially lower in magnitude than the lowest value of input voltage at which internal circuitry of the boost circuit is operable.